The cathodic protection of metal sections of structures is well known. This technique provides corrosion protection for the metal section by the formation of an electrical circuit that results in the metal section acting as a cathode and therefore oxidation of the metal is restricted.
An anode is generally an electrode that supports a substantial net oxidation reaction. A cathode is generally an electrode that supports a substantial net reduction reaction. A metal surface may contain both anodic areas and cathodic areas. A spontaneous corrosion cell occurs on a metal surface that supports both anodic and cathodic reactions (oxidation and reduction reactions respectively). A cell of a battery is typically an isolated chemical reactor that spontaneously generates a potential difference between its positive and negative electrodes (its cathode and anode respectively). Cathodic protection is an electrochemical treatment for steel in concrete.
One known type of system for cathodic protection is an impressed current system, which makes use of an external power supply, either mains or battery, to apply current to the metal section to be protected so as to make it a cathode. These systems generally require complex circuits to apply the current appropriately and control systems to control the application of the current. Furthermore, those that are supplied with mains power clearly can encounter difficulties with power supply problems such as power surges and power cuts, whilst those powered by battery have to overcome the issue of locating the battery at an appropriate position, which both allows the battery to function correctly and supports the weight of the battery.
Such impressed current systems may have a battery secured to the exterior of the structure containing the metal sections to be protected, which clearly adversely affects the look of the structure.
Other systems for cathodic protection, which avoid the need for bulky or complex components (e.g. power supplies and electrical wiring) make use of a sacrificial anode coupled to the metal section. The sacrificial anode is a more reactive metal than the metal of the metal section and therefore it corrodes in preference to the metal section, and thus the metal section remains intact. This technique is commonly used in the protection of steel reinforcements in concrete, by electrically connecting the steel to a sacrificial anode, with the circuit being completed by electrolyte in the pores of the concrete. This system is termed a sacrificial or galvanic system.
The anodes used in the impressed current system are usually inert anodes comprising carbon or titanium. In these anodes the anodic reaction substantially comprises the conversion of water into oxygen gas and acid. By contrast, the anodic reaction on sacrificial anodes substantially comprises the dissolution of the sacrificial metal element. The advantage of sacrificial anodes is that they can be used without a power supply, but the disadvantage is that they are consumed. They therefore are not generally used in impressed current systems. A well known exception to this occurs with zinc or aluminium alloys that are thermally sprayed as a coating onto the concrete surface and are used with a power supply. While these anodes are consumed in the process of delivering protection, they are readily replaced because they are applied to an exposed concrete surface. However, sacrificial metal dissolution occurs at the interface between the sacrificial anode and the electrolyte. As a result sacrificial anodes applied directly to the concrete surface often exhibit adhesion problems.
Anodes for sacrificial systems include compact discrete zinc anodes in contact with a purpose designed backfill embedded in cavities within the concrete and thermally applied coatings of zinc and aluminium applied to the concrete surface. Surface applied sacrificial anodes exhibit adhesion problems, while embedded compact discrete anodes lack the power to arrest an aggressive corrosion process because they have to drive more current through a small volume of concrete near the anode. The effective anode circuit resistance of embedded compact discrete anodes is high relative to surface applied sacrificial anodes.
Examples of problems with both impressed current and sacrificial anode systems are provided in a Virginia Transportation Research Council report number VTRC 07-r35 entitled “Survey of Cathodic Protection Systems on Virginia Bridges” dated June 2007 and available from http://www.virginiadotorg/vtrc/main/online_reports/pdf/07435.pdf.
A problem associated with sacrificial cathodic protection arises from the fact that it is the galvanic voltage between the sacrificial anode and the metal section that drives current through the electrolyte between these components. This voltage is limited by the natural potential difference that exists between the metal section and the sacrificial anode. Accordingly, the higher the resistance of the electrolyte, the lower the current flow is across the electrolyte between a given metal section and sacrificial anode, and hence the application of sacrificial cathodic protection is restricted.
Protection of the steel reinforcements is in particular required when chloride ions are present at significant concentrations in the concrete, and therefore cathodic protection is widely used in relation to concrete structures in locations which are exposed to salt from road de-icing or from marine environments.
There is a need for a sacrificial anode arrangement that can give rise to a voltage between itself and the metal section greater than the natural potential difference that exists between the metal section and the material of the sacrificial anode wherein the anode is strongly attached to the concrete structure.